Three different topics have been studied under this project: (a) We have studied the effect of counterions on the rigidity of DNA molecules in solution using a charged worm-like model in which the electrostatic charge-charge interactions are treated explicitly based on Manning's condensation theory and found that counterion has a profound effect on the nonelectrostatic persistence length of DNA. (b) We have obtained the exact formulas for relase, uptake and pulse chase of labeled subunits of F-actin and microtubules that undergo head-to-tail polymerization. (c) We have presented a new way of plotting the isotherm for ligand binding to linear biopolymers, which supplements the defects of conventional plots.